1. Field of the Invention
The present invention is directed to an implantable cardiac stimulation device of the type which is operable in multiple modes respectively based on different algorithms.
2. Description of the Prior Art
Implantable cardiac stimulation systems generally include one or more stimulation and sensing electrodes, arranged on one or many insulated electrode leads, and a pacemaker housing having inter alia, a control unit and pulse generating means.
The one or more electrodes are used to provide electrical stimuli directly to the heart muscle. These stimuli can be pacing pulses, and can sometimes include relatively larger shocks, such as are used to break up tachyarrhythmias. The electrodes may also be used for sensing the intracardial electrogram (IEGM). In addition to these electrodes, and to further enhance the pacemaker therapy, sensors may be used by the device to sense blood gases, respiration, cardiovolume, temperature, pressure or other physiological conditions.
The pacemaker housing is normally implanted subcutaneously in the region of the clavicle. The insulated electrode lead or leads are inserted into the heart in accordance with the normal procedure that is well known to those skilled in the field of pacemakers and are then connected to the housing.
The control units of modem devices are sophisticated and include control logic circuits, timing circuits, and input/output (I/O) circuitry that connects the control logic with the electrodes and/or sensors. For example, the I/O circuit provides analog-to-digital and digital-to-analog conversion, and provides the desired electrical stimuli as pulses of the desired amplitude, duration and frequency. The control unit typically includes a microprocessor and memory, and is also configured to allow remote programming after implantation in the patient""s body.
Early pacemakers were fixed-rate devices providing electrical stimuli to the heart if the heart failed to beat within a predetermined time period. However, microprocessor-based technology has enabled implantable devices to make complex logical decisions based on a variety of physiological data. As examples, modem implantable devices have the ability to distinguish between different types of tachyarrhythmia and to select an appropriate therapy that does not impose undesired trauma on the heart. The present day microprocessor-based devices are capable of distinguishing normal physiological conditions from pathological conditions and also of selecting between alternative therapies for the latter. Logical decisions based on physiological variables, therapies responsive to different heart conditions, and automatic self-configuration are examples of what is referred to as automaticity.
As the pacing algorithms grow more sophisticated and complex the algorithm code itself will also grow in size and complexity and thus require a high degree of operational safety.
U.S. Pat. No. 5,633,735 assigned to Pacesetter, Inc., discloses a device capable of operation in three different modes. The device is capable of detecting software errors and in the case of such detection switches from the first mode to the second mode of operation. Each of the first and second modes is able to function automatically. If another software error is detected the device switches to the third mode which is fixed-rate pacing. The device is not capable of switching back to the first or second mode. The error-detecting mechanism can detect software-errors such as parity error, watchdog error, checksum error etc.
U.S. Pat. No. 4,467,810 discloses a multi-mode microprocessor-based programmable cardiac pacemaker.
Software algorithms supporting new additional pacing functions must be able to cooperate with a basic or existing algorithm in a safe and reliable way.
Accordingly, there is a definite need for an implantable device, which is capable of supporting new pacing algorithms, as well as being functional as a well-established and well-tested pacemaker.
An object of the invention is to provide an implantable cardiac stimulation device offering a high degree of safety for a microprocessor-based device having two or more pacing algorithms.
The above object is achieved in accordance with the principles of the present invention in an implantable cardiac stimulation device having a first pacing algorithm which is executable by a microprocessor and which is capable of independently administering stimulation therapy to a patient""s heart, and wherein a second microprocessor-executable pacing algorithm is provided, the first and second pacing algorithms both actively generating pacing parameters during each cardiac cycle, and wherein the pacing parameters of the second pacing algorithm are only able to result in an actually-administered stimulation therapy if these parameters fall within parameter ranges which are calculated to be allowable for stimulation by the first pacing algorithm.
Thus, improved safety is obtained by allowing two or more pacing algorithms, including a first algorithm, during each heart cycle to actively generate pacing parameters, but only allowing a request for stimulation therapy from one of the other algorithms if this request falls within parameter ranges calculated to be allowable for stimulation by the first algorithm.
A pacing algorithm is in this context regarded as a set of instructions capable of performing stimulation therapy by generating instructions to pulse generating circuits.